Gas phase processes for the homopolymerization and copolymerization of monomers, especially olefin monomers, are known in the art and are typically conducted, for example, by introducing the gaseous monomer or monomers into a stirred and/or fluidized bed of resin particles and catalyst. In a fluidized-bed polymerization of olefins, the polymerization is conducted in a fluidized-bed reactor, wherein a bed of polymer particles is maintained in a fluidized state by means of an ascending gas stream including gaseous reaction monomer. The polymerization of olefins in a stirred-bed reactor differs from polymerization in a gas fluidized-bed reactor by the action of a mechanical stirrer within the reaction zone, which contributes to fluidization of the bed. As used herein, the term “fluidized-bed” also includes stirred-bed processes and reactors.
During the course of polymerization, fresh polymer is generated by the catalytic polymerization of the monomer, and polymer product is withdrawn to maintain the bed at constant volume. An industrially favored process employs a fluidization grid to distribute the fluidizing gas to the bed, and also to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via one or more discharge conduits disposed in the lower portion of the reactor, near the fluidization grid. The fluidized bed includes a bed of growing polymer particles, polymer product particles and catalyst particles. This reaction mixture is maintained in a fluidized condition by the continuous upward flow from the base of the reactor of a fluidizing gas which includes recycle gas drawn from the top of the reactor, together with added make-up monomer. The fluidizing gas enters the bottom of the reactor and is passed, preferably through a fluidization grid, upwardly through the fluidized bed.
The polymerization of olefins is an exothermic reaction, and it is therefore necessary to cool the bed to remove the heat of polymerization. In the absence of such cooling, the bed would increase in temperature until, for example, the catalyst became inactive or the polymer particles melted and began to fuse, causing fouling. In the fluidized-bed polymerization of olefins, a typical method for removing the heat of polymerization is by passing a cooling gas, such as the fluidizing gas, which is at a temperature lower than the desired polymerization temperature, through the fluidized-bed to conduct away the heat of polymerization. The gas is removed from the reactor, cooled by passage through an external heat exchanger and then recycled to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidized-bed at the desired polymerization temperature. In this method of polymerizing alpha olefins, the recycle gas generally includes one or more monomeric olefins, optionally together with, for example, an inert diluent gas or a gaseous chain transfer agent such as hydrogen. The recycle gas thus serves to supply monomer to the bed to fluidize the bed and to maintain the bed within a desired temperature range. Monomers consumed by conversion into polymer in the course of the polymerization reaction are normally replaced by adding make-up monomer to the recycle gas stream.
The polymerization process can use Ziegler-Natta and/or metallocene catalysts. A variety of gas phase polymerization processes are known. For example, the recycle stream can be cooled to a temperature below the dew point, resulting in condensing a portion of the recycle stream, as described in U.S. Pat. Nos. 4,543,399 and 4,588,790. This intentional introduction of a liquid into a recycle stream or reactor during the process is referred to generally as a “condensed mode” operation. Further details of fluidized bed reactors and their operation are disclosed in, for example, U.S. Pat. Nos. 4,243,619, 4,543,399, 5,352,749, 5,436,304, 5,405,922, 5,462,999, and 6,218,484, the disclosures of which are incorporated herein by reference.
The properties of the polymer produced in the reactor are affected by a variety of operating parameters, such as temperatures, monomer feed rates, catalyst feed rates, particle size and hydrogen gas concentration. In order to produce polymer having a desired set of properties, such as melt index and density, polymer exiting the reactor is sampled and laboratory measurements carried out to characterize the polymer. If it is discovered that one or more polymer properties are outside a desired range, polymerization conditions can be adjusted, and the polymer resampled. This periodic sampling, testing and adjusting, however, is undesirably slow, since sampling and laboratory testing of polymer properties such as melt index, molecular weight distribution, average particle size, particle size distribution, and/or density is time-consuming. As a result, conventional processes can produce large quantities of “off-spec” polymer before manual testing and control can effectively adjust the polymerization conditions. This occurs during production of a particular grade of resin as well as during the transition process between grades.
Methods have been developed to attempt to provide rapid assessment of certain polymer properties and rapid adjustment of polymerization conditions. U.S. Pat. No. 7,116,414 discloses a method of on-line monitoring using Raman based methods. PCT publications WO 2001/09201 and WO 2001/09203 disclose Raman-based methods using principal components analysis (PCA) and partial least squares (PLS) to determine concentrations of components in a slurry reactor. The concentration of a particular component, such as ethylene or hexene, is determined based on measurements of a known Raman peak corresponding to the component. U.S. Pat. No. 5,999,255 discloses a method for measuring a physical property of a polymer sample, preferably nylon, by measuring a portion of a Raman spectrum of the polymer sample, determining a value of a preselected spectral feature from the Raman spectrum, and comparing the determined value to reference values. WO 2005/049663 discloses on-line measurement and control of polymer properties using Raman spectroscopy in a fluidized bed reactor.
Additional background information can be found in U.S. Pat. Nos. 6,144,897 and 5,151,474; European Patent application EP 0 561 078; PCT publication WO 98/08066; and Ardell, G. G. et al., “Model Prediction for Reactor Control,” Chemical Engineering Progress, American Institute of Chemical Engineers, U.S., vol. 79, no. 6, Jun. 1, 1983, pages 77-83 (ISSN 0360-7275).
It would also be desirable to have methods of controlling a gas-phase fluidized bed reactor to maintain desired polymer properties, based on a rapid, on-line determination of the polymer properties, such as average particle size and particle size distribution.
The average particle size and particle size distribution of polymer particles formed in a gas phase reactor are known to affect the performance and robustness of the process. Smaller particles contribute to detrimental polymer carry-over into the recycle system and static charges within the reactor. In addition particle size serves as indirect measure catalyst productivity. Therefore, measuring the particle size in general is useful in monitoring and controlling the performance of a gas phase reactor. A method that affords an indication of particle size in real-time and in-situ has the added benefits of a faster response time and no sample preparation. The current method of particle size measurement relies on reactor sampling and offline sieve analysis which results in a time lag, and therefore a potential difference, between what the process is actually producing and what is measured.
WO 2002/054061 discloses a method for determining flour baking properties using near infrared (NIR).
WO 2005/005965 discloses the use of a non-Gaussian laser beam to measure particle size and particle concentration in dilute media.
Analyst, 2003, 128 (11), 1326-1330 discloses a method for determining percentage volume particle size distribution of powdered microcrystalline cellulose.
U.S. Pat. No. 6,864,331 discloses a polymerization process where a sensor probe connected to a near-IR spectrophotometer light source connected by a fiber optic cable is used to measure a polymer characteristic. The value for the characteristic is a component of an algorithm and the algorithm is used, in real time, to monitor and/or control the process for preparing a polymer. U.S. Pat. No. 6,864,331 focuses on slurry processes and the settling legs on a loop reactor.
WO 2005/005965 discloses the use of a non-Gaussian laser beam to measure particle size and particle concentration in dilute media.
Macromolecular Reaction Engineering (2011), 5(3-4), 150-162 discloses the use of NIR to track the evolution of particle growth during the emulsion polymerization of styrene.
Polymer Engineering and Science (2011), 51(10), 2014 2024-2034 discloses the use of NIR for the in-situ, real-time monitoring of the emulsion polymerization of methyl methacrylate.
Chimia (2001), 55(3), pg 231-233 discloses a method for direct observation of growing polymer particles in a gas phase polymerization cell with a transparent lid using IR.
Additional references of interest include: U.S. Pat. No. 7,329,547.